A BART police officer struggling to handcuff a 22-year-old man stood up over the facedown Hayward resident and fired a single shot into his back while a handful of officers watched, a video taken by a train passenger apparently shows.

The attorney for the family of Oscar Grant III, fatally shot by an unidentified BART officer early New Year's Day, said Sunday he plans to file a $25 million lawsuit against the department and asked prosecutors to consider filing murder charges against the officer.

The shooting occurred shortly before 2 a.m. Thursday after five officers responded to the Fruitvale station to reports of a fight on a train, officials said, though they have not confirmed whether Grant was involved in the fight.

The new video, obtained by television station KTVU, shows two officers restraining a struggling suspect. While the man is lying face down on the ground, one officer appears to be seen pulling out a gun and firing a single shot into his back.

Civil rights attorney John Burris, known for his work in several high-profile cases involving police abuse and corruption, said at a Sunday news conference that the shooting was "the most unconscionable" he has ever seen. He said the Alameda County district attorney should consider filing charges of second-degree murder or manslaughter against the officer.

"I've drafted a notice of claim against BART for $25 million I plan to submit officially," Burris said, adding thatthe officer had violated Grant's civil rights and wrongfully caused his death.

The Police Department is in the early stages of a thorough investigation, BART police Chief Gary Gee said Sunday at a news conference. He declined to discuss many details, as doing so "before all the facts are in could compromise individual recollections and do disservice to the truth and the answers we're all seeking."

BART police are cooperating fully with a parallel investigation by the Alameda County district attorney's office, Gee said.

Gee declined to identify the officer but said he is a two-year BART police veteran. The officer was given drug and alcohol tests before being sent home on administrative leave Thursday, Gee said.

The last BART officer-involved shooting occurred in May 2001, Gee said.

Mario Pangelina Jr., whose sister had a 4-year-old daughter with Grant, said he was on the same train as Grant that night, but on a different car. He said he saw Grant's interactions with police immediately before the shooting.

"First, an officer grabbed Oscar by the neck and pushed him against the wall," Pangelina said. "Oscar didn't fight him, but he didn't go down either. He was like, 'What did I do?' Then another officer came up with his Taser and held it right in his face. Oscar said, 'Please don't shoot me, please don't Taser me, I have a daughter,' over and over again, real fast, and he sat down."

Grant was the only man in a small group sitting against the wall who was not handcuffed, Burris said, so officers grabbed him away from the wall and pressed him belly-down onto the ground.

"One officer was kneeling over his neck and head, and another standing over him," Burris said. "He was not kicking, and one officer was pulling on his arm. The standing officer pulled out his weapon and, within moments, fired the gun into Mr. Grant's back."

Burris said the bullet went through Grant's lower back and ricocheted off the ground up into his lungs, killing him.

BART's 206 sworn officers attend the same academies and training programs as city police and sheriff's deputies. According to BART's Web site, its requirements go beyond state guidelines, as every officer applicant must have completed at least a year of college.

Police have one video of the incident in evidence, different from the video that local media have released, and the quality of that video makes it hard to reach a sure conclusion, Gee said.

"It's not clear to me why the officer felt he needed to shoot. I don't know, and from my perspective it doesn't matter," Burris said.

Two authorities on police use of deadly force, both former law enforcement officers, said the newly discovered tape leaves unanswered questions.

"Strictly on the basis of this video, it is impossible to determine whether the shooting was justified because the officer who fired the shot might have seen some imminent threat to his or others' lives that the camera does not detect at that distance, angle and resolution," said Michael Scott, a University of Wisconsin law professor, former police chief in Florida and co-author of "Deadly Force: What We Know."

Scott said he watched the video several times. If there was a threat, he wrote in an e-mail to the Times, it "would most likely have to be a firearm or other weapon in the possession of Mr. Grant. However, if it turns out that Mr. Grant had no such weapon, it is awfully difficult to imagine what might have justified the use of deadly force."

Curtis Cope agreed that the tape doesn't show enough to draw clear conclusions.

"There are so many things we don't know," said Cope, a former 30-year law enforcement officer who has conducted police training and provides expert testimony in police procedure cases. "We certainly don't know the reason why they decided to put him prone on the ground. We don't know what reactions were taking place, what orders were being given and whether or not he is then complying or not complying. ... You need to look at every possible angle of it. Those angles all take time."

Grant was a butcher at popular Oakland grocery store Farmer Joe's and a loving father, family members said Sunday.

"He was so happy with his daughter," said Lita Gomez, sister to the mother of Grant's child. "You could see he was just so happy when he looked at her. Now, he's not going to be there for kindergarten. He's not going to be there for her prom. He's not going to be there for her wedding. She was robbed of that."

Family members erected a memorial for Grant outside the Fruitvale BART station Saturday night, where they said they plan to continue honoring his memory for 10 days.

A public funeral service is planned for 11 a.m. Wednesday at Palma Ceia Baptist Church, 28605 Ruus Road in Hayward, family members said.

Gee asked anyone with information on the shooting to call BART investigators at 877-679-7000, ext. 7040, or the Alameda County district attorney's office at 510-272-6222.